1. Field of the Invention
This invention relates generally to aerodynamic surfaces and, more particularly, to improved constructions and control schemes for such aerodynamic surfaces which provide for aerodynamic control and for significant reductions in noise in the case of rotor blades.
2. Description of Related Art
Commercial and military helicopters/tiltrotors in hover and in high-speed forward (or edgewise) flight generate an impulsive noise signature which is commonly referred to as high-speed impulsive (HSI) noise. Three factors are known to affect the intensity of HSI noise. First, the strength of the shock wave(s) terminating the local pocket(s) of the supersonic flow on the blade affects the HSI noise. The unsteady chordwise motion of the shock wave as a function of blade azimuth also affects the HSI noise. The third factor contributing to HSI noise is the local airfoil geometry, such as the maximum thickness and camber constituting the tip of the rotor blade. The local airfoil geometry is known to effect the chordwise extent of the local pockets of supersonic flow. One or more of the above factors can be altered using an active or passive noise control technique in order to reduce or eliminate HSI noise.
In forward flight, HSI noise is typically generated at high advance ratios predominantly from the advancing rotor blades where local region(s) or pockets of supersonic flow are most likely to occur. At high advance ratios, the supersonic flow regions are usually terminated by strong shock waves having strengths that are proportional to the static pressure rises across the shock waves or, alternatively, to the peak local Mach numbers ahead of the shock waves. In forward flight, the strengths of the shock waves and the chordwise extents of the supersonic flow pockets vary with the azimuthal positions of the rotor blade due to the variations in the local free stream Mach numbers. At much higher advance ratios, the local pockets of supersonic flow, now associated with stronger shock waves on the rotor blade, extend beyond the tip of the blade sometimes exceeding what is commonly defined as the "sonic" cylinder. Beyond the radius which defines the sonic cylinder, the flow is entirely supersonic. At these conditions, the flow on the rotor blade is referred to as being delocalized.
In hover, HSI noise can also occur as a result of a number of factors including a high rotational tip Mach number, a combination of a moderate tip Mach number and a relatively thick blade tip airfoil section, and a combination of a moderate tip Mach number, a relatively thin blade tip airfoil and high tip twist.
Regardless of the mode of operation, once strong shock waves have formed, HSI noise can severely impact the operation of both military and, to a lesser extent, commercial rotorcraft. This noise source is especially important for military operations, since it is known to be responsible for aircraft detection. A reduction in the intensity of HSI noise and/or the manipulation, using active or passive control techniques, of the resulting noise signature, is needed.
No active noise control techniques are known for reducing the strength of the shock waves responsible for HSI noise. Conventional methods for reducing HSI noise rely predominantly on passive, indirect approaches, which demand the highest possible drag divergence Mach number and the minimum allowable thickness for the rotor blade tip airfoil. Conventional methods, although partially effective, do not guarantee low HSI noise levels at operation conditions other than the specifically designed and engineered operation condition.